NA VI G A TION. 47 



other causes ; and when, at the same time, either 

 the ship is altogether out of sight of land, or 

 the shore presents no distinct objects by which 

 to fix her position, recourse may be had to the 

 ground log. This is a small lead, with a line 

 divided like the log-line, the lead remaining 

 fixed at the bottom ; the line exhibits the effect 

 of the combined motion of the ship through the 

 water, and that of the water itself, or the current ; 

 and therefore the course (by compass), and distance 

 made good are obtained at once. 



34. "THE GLASSES. The long glass runs out 

 in 3O S or in 28 s ; the short glass runs out in 

 half the time of the long one. 



" When the ship goes more than five knots, the 

 short glass is used, and the number of knots shown 

 is doubled. 



"The sand-glasses should frequently be ex- 

 amined by a seconds watch, as in damp weather 

 they are often retarded, 1 and sometimes hang 

 altogether. One end is stopped with a cork, which 



1 Why is the glass not hermetically sealed so that the sand put 

 in dry may remain dry for ever ? (W. T.) 



